Proposal of a new equation to estimate globe temperature in an urban park environment

Globe temperature is one element of the heat stress index, the Wet Bulb Globe Temperature, which is used to evaluate how radiation adds to thermal discomfort in the workplace. As the measurement of globe temperature is not standardized, empirical equations were introduced to estimate the globe temperature from weather factors, including air temperature, solar radiation and wind speed. As it was not known whether these equations were applicable in an urban park environment with vegetation, we observed the globe temperature using a set of instruments in three parks. The observation along with the heat balance analysis of the globe revealed that the globe temperature depended curvilinearly on solar radiation and that wind speed influenced this dependence. We compared two previously proposed empirical equations to the observed globe temperature and found both equations had systematic estimation errors. Although the errors were reduced by fitting the equations to the observed data and reevaluating their numerical constants, the equations still had shortcomings, as one did not consider wind speed and the other included a discontinuity. We therefore derived a new equation based on the heat balance equations of the globe with its numerical constants experimentally determined. This equation was able to predict the curvilinear dependence of the globe temperature on global solar radiation without any discontinuity, and it also showed the globe temperature response to wind speed.